Martensitic stainless steel for high temperature applications

ABSTRACT

A martensitic stainless steel for high temperature applications consisting essentially of about .15 to .35 percent carbon, up to 1 percent manganese, up to 1 percent silicon, 11 to 13 percent chromium, .25 to 1.25 percent nickel, .50 to 1.50 percent molybdenum, .50 to 1.50 percent tungsten, .10 to .50 percent vanadium, .05 to .50 percent niobium and the balance substantially all iron, characterized in having good impact strength, a lack of any substantial delta ferrite and high creep rupture life in excess of about 40,000 pounds per square inch at 100,000 hours and 1000*F.

United States Patent n91 Hahn [ MARTENSITIC STAINLESS STEEL FOR HIGH TEMPERATURE APPLICATIONS [75] lnventor: Ronald A. Hahn, Lower Burrell, Pa.

[73] Assignee: Allegheny Ludlum Industries, Inc.,

Pittsburgh, Pa.

[22] Filed: Feb. 1, 1972 [21] Appl. No.: 222,502

1 1 Oct. 23, 1973 Primary ExaminerHyland Bizot Attorney-Vincent G. Gioia et a1.

[57] ABSTRACT A martensitic stainless steel for high temperature applications consisting essentially of about .15 to .35

if 75/128 75,128 gg ggl percent carbon, up to 1 percent manganese, up to 1 w v percent silicon, 11 to 13 percent chromium, .25 to 1 0 can G 1.25 percent nickel, .50 to 1.50 percent molybdenum, .50 to 1.50 percent tungsten, .10 to .50 percent vanadium, .05 to .50 percent niobium and the balance sub- [5.6] References Clted stantially all iron, characterized in having good impact UNITED STATES PATENTS strength, a lack of any substantial delta ferrite and' 3,139,337 6/1964 Boyle 75/128 G high creep rupture life in excess of about 40,000 3,113 5/1957 Rait pounds per square inch at 100,000 hours and 1000F. 2,801,916 8/1957 Harris..... 2,968,549 l/1961 Brady 75/128 G 4 Claims, 1 Drawing Figure TYPE 422 40, 000 e Nb l (r, 3 Q: 30,000 k (I) TYPE 422 TYPE 4/9 20,000 i l l l LARSE/V -M/LLEP PARAMETER PAIENIEDBBI 23 M3 3 7 67 390 7' YPE 422 STRESSPS/ TYPE 422 TYPE 4/9 20,000 I l l 1 L 42 45 44 45 46 47 LARSE/V -M/LLER PARAMETER MARTENSITIC STAINLESS STEEL FOR HIGH TEMPERATURE APPLICATIONS BACKGROUND OF THE INVENTION While not limited thereto, the alloy of the present invention is particularly adapted for use in high temperature applications such as steam turbine blades and the bolts which hold together the two halves of the steam turbine casing. As attempts are made to operate turbines at higher temperatures, the need arises for alloys having improved creep rupture strengths. This is particularly true of elements such as the bolts which hold the two halves of the turbine casing together. As the size of the turbine is increased, the pressure on the bolts also increases. This requires that more or larger bolts of presently used alloys, such as Type 422, must be used. However, it has become difficult to increase the number and size of the bolts because of the restricted amount of area available on the flanges of the casing halves. What is needed is an alloy with higher creep rupture strength characteristics.

While age hardening austenitic super. alloys offer much higher strengths in the temperature range of 900 to 1 100F than Type 422 alloys, for example, this higher strength cannot be utilized since the austenitic super alloys have coefficients of expansion sufficiently higher than that of the high strength low alloy casing, meaning that the bolts will loosen upon heating up to operating temperature. What is needed is a higher strength martensitic alloy.

In the past, a number of 12 percent chromium martensitic stainless steel alloys containing intentional ad ditions of niobium have been developed in an effort to enhance the strength and other physical properties of the alloys. For example, U. S. Pat. No. 3,000,729 teaches that niobium enhances the strength of a 12 percent chromium martensitic steel and at the same time improves the impact properties of the alloy. U.S. Pat. No. 2,513,935 discloses a steel having high creep strength and improved scaling resistance over Type 410 and Type 410 containing additions of the molybdenum and tungsten. U.S. Pat. No. 2,469,887 reveals AISI Type 410 steel containing nickel and niobium with improved rupture strength. U.S. Pat. No. 3,389,91 1 discloses a steel having high strength, resistance to overtempering and high impact strength, higher than those of the higher carbon Type 422. However, the rupture strength of this steel is inferior to that of niobium-containing grades.

Other alloys which contain niobium and additional alloying elements have also been developed, such as that shown in the U.S. Pat. No. 3,139,337 which contains chromium, molybdenum, vanadium, niobium and nitrogen. None of these chromium martensitic steels, however, is entirely satisfactory for turbine bolt and the like applications.

SUMMARY OE THE INVENTION In accordance with the present invention, an improved martensitic stainless steel is provided containing about 12 percent chromium and critical amounts of niobium. Specifically, the invention resides in the discovery that by adding to Type 422 martensitic stainless steel up to 0.50 percent niobium and preferably about 0.03 percent niobium, an alloy results which is characterized in having an exceptionally good impact strength, a lack of any substantial delta ferrite, and a creep rupture life in excess of 40,000 pounds per square inch at 100,000 hours and 1,000F. Surprisingly, it has been found that when niobium is added to other similar martensitic stainless steels, the desirable results of the invention do not result. Either the other physical properties of the alloy are diminished or the creep rupture characteristics are inferior to those of Type 422 with the addition of niobium. For example, the addition of niobium to Type 419 martensitic stain less steel (the composition of which is given hereinafter) does give improved creep rupture characteristics, perhaps even superior to those of the alloy of the present invention. However, the addition of niobium to Type 419 reduces its impact strength and results in the formation of delta ferrite, which makes the alloy brittle and unsatisfactory for bolts and other similar elements to be used in high temperature applications.

The foregoing and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying single FIGURE drawing which is a plot illustrating the comparative creep rupture properties of Type 419 and 422 martensitic stainless steels with and without the addition of niobium. I

The steel of the invention has the following broad and preferred ranges of composition:

Phosphorus, sulfur and nitrogen are included as incidental impurities; however it is desirable to reduce the sulfur content as low as possible in order to minimize brittleness. In Table I, the minimum niobium addition is shown as 0.05 percent by'weight; however it should i be understood that any amount of niobium under 0.50

percent and effective to produce improved creep rupture strength characteristics is within the scope of the invention.

In order to show the superior characteristicsof the alloy of the invention, heats of Types 422 and 419 martensitic stainless steels with and without the addition of niobium were prepared, the composition of the heats being shown in the following Table II. Heat MQ- comprises a conventional Type 422 stainless steel with the addition of about 0.29 percent niobium; whereas Heat KK-33 comprises Type 419 stainless steel with the addition of .52 percent niobium.

TABLE II.COMPOSITION OF ALLOYS SHOWING OUR INVENTION Type Heat No C Mn P S Si Cr Ni Mo W V Nb N 422 KD22-B 0.24 0.37 0.010 0.011 0.36 12.04 077 1.03 1.02 0.32 0.074 MQ-79 2.5 .41 .008 .006 .38 12.08 .75 .98 .96 .33 .001 .080 MQ-80 .25 .41 .008 .006 .39 12.13 .73 .98 1.04 .29 .29 .074 419 KF-52 .23 .47 .011 .013 .30 11.81 1.46 .57 3.20 .35 .073 419+Nb KK-33 .23 .58 .013 .004 .36 11.86 1.60 .50 3.05 .32 .52 .080

Type 422 without Nb. 2 Type 422 without Nb. Type 422 with Nb.

4 Type 419 without Nb. Type419 with Nb. 7

Themiechanical properties of the heats of Table II are shown in Table 111.

TABLE [IL-MECHANICAL PROPERTIES OF ALLOYS SHOWING OUR INVENTION Ultimate Hardness .02 yield .2 yield yield Elong. Reduction (VN Type Heat No. Treatment Hrs (Rc) strength strength strength (2". of area (%1 (ft-lbs.)

422 KD-22-B 2000' 6 +12s0 4 34.0 111,800 126.050 152.450 15.8 49.0 28. 2a.

422 MQ-79 Z000 O Q 1250 1250 2 2 36.0 113,810 129,565 158,080 16.0 47.11

422 Nb MQ-80 2000 CO +1250 1250 2 2 36.0 114,140 135,550 162,260 15.5 48.6 25.5.

419 KF-52 2000 0Q 1250 4 36.0 101.100 130,600 1 6 8,0 00 14.3 39.5 l7, 19.

419 Nb KK-33 2000 CO 1250 4 37.0 110,450 141,050 170,850 14.3 47.0 11.5, 14.5.

' The hardening time was minutes for all heat treatments. OQ oil quench.

AQ air quench.

2 Heating time after quench, followed by air cool.

-' Charpy V-notch test.

The properties show that Type 422 stainless steel with the addition of niobium has a higher strength than the same steel without the addition of niobium (i.e., Heat MQ-79) at both the 2,000F and 1900F hardening temperatures. The hardening times were 30 minutes in all cases with the tempering temperatures and times as shown by the third and fourth columns of Table 111. It was found that the addition of niobium does not materially vary hardness; however the higher strength for the same hardness is accompanied by a very slight drop in elongation with a slight increase in reduction of area. This shows that the addition of niobium has not reduced the ductility of the alloy when tensile tested. The

alloys were also impact tested using the Charpy V- notch test. It was found that the impact strength of Heat MQ-8O (with the niobium addition) decreases somewhat from that for Type 422 without the addition of niobium; however this is perhaps due to the heat treatments employed. As can be seen from Heats KF-52 and KK-33 in Table III, the addition of niobium to Type 419 stainless steel drastically reduces the impact strength; and it is for this reason that Type 419 stainless steel is not a good base material from which to derive an alloy having the desirable characteristics of the invention.

The advantage of adding niobium to Type 422 stainless steel is shown in the following Table IV and the-accompanying drawing.

TABLE IV.RUPTURE PROPERTIES OF ALLOYS SHOWING OUR INVENTION Stress to rupture Applied Time to L-M parameter Type Heat No. Treatment Hrs. Re Temp. stress# Elong. R.A. rupture C-Z5 43.8

422 KD-22-B 2000 0Q 1250 4 1,200 26,000 26.9 78.6 88.6 44.7 32.600 1,100 44,000 19.9 68.3 198.8 42.6 1900 0Q 1250 4 1,200 26,000 29.0 81,5 60.7 44.5 30,800 1,100 44,000 26.9 77.2 173.0 42.5

422 MQ-79 2000 AQ 1250 1250 2 2 33.0 1,200 26,000 28.8 86.4 94.6 44.8 34,000 1.100 44,000 22.4 82.0 257.0 42.8 1,100 56,000 39.3 83.6 2.4 39.6 1900 0Q 1250 1250 2 2 35.0 1,200 26,000 32.7 88.2 30.7 44.0 27,000 1,100 44,000 38.7 80.6 0.7 38.7 1,100 56,000 32.8 81.4 2.0 39.5

422 Cb MQ-80 2000 00 1250 1250 2 2 36.0 1,200 26,000 25.9 78.0 933.3 46.2 44,000 1,100 44,000 18.0 68.8 1,200.0 43.8 1,100 56,000 26.0 74.9 57.5 2000 0Q 1250 1250 2 2 1.200 26,000 19.0 15.2 565.1) 1,100 44,000 22.4 73.9 1,017.6 1,100 56,000 21.11 73.11 238.11 1900 00 1250 1250 2 2 35.0 1,200 26,000 15.) 76.6 343.6 1,100 44,000 25.1 72.5 742.1

419 KF52 2000 Q 1250 1.200 26.000 23.0 50.2 11111.4 1.100 44.000 14.0 50.11 589.4 2000 0Q 1300 1,200 26,000 24.4 63.8 175.6 1,100 44,000 17.2 57.4 411.1 1900 0Q 1250 4 1,200 26,000 34.9 76.9 106.8 1,100 44,000 18.9 63.9 401.4

"creep rupture strengths in th e last column of Table IV were derived by averaging the results attained from at least two specimens heated to different temperatures and subjected to different stresses for different times. The results were obtained from the standard Larson- Miller equation with 25 as the constant and 43.8 as the parameter. The results for different parameters are plotted in the drawing. Note that heat MQ-79 (Type 422 without niobium) has a stress rupture life of 34,000 when hardened at 2,000F; whereas heat MQ-80 (Type 422 with niobium) has a stress rupture life of 44,000, an increase of about 31 percent. When the same heats were hardened at 1900F, the normal hardening temperature for Type 422, the increase is 56 percent for the 1250 1250, 2+2 data. Of course, even higher creep rupture strengths are achieved with heat KK-33 (Type 419 with niobium); but this suffers from reduced impact strength as shown in Table III.

Accordingly, best results are achieved to increase creep rupture life without sacrificing impact strength by adding 0.05 to 0.50% by weight of niobium to a Type 422 martensitic stainless steel.

Although the invention has been shown in connection with certain specific embodiments, it will be readily apparent to those skilled in the art that various TABLE IV.RUPTURE PROPERTIES OF ALLOYS SHOWING OUR INVENTION -(0riiinu0tl TN-w MW 7 I Stress to rupture Applied Time to L-M parameter Type Heat N0. Treatment Hrs. Rc Temp. stress# Elong. R.A. rupture C-25 43.8 i 119 4 Ch K1033 m T2 5 4 1,200 26,000 21.6 53.4 I 7 59.? *Z5T;""4E;bb6"

nadium, 0.05 to 0.50 percent niobium, and the remainder substantially all iron; said steel having a rupture stress in excess of 40,000 pounds per square inch at a time of 100,000 hours and a temperature of 1000F.

2. The martensitic stainless steel of claim 1 containing about 0.3 percent niobium.

3. The martensitic stainless steel of claim 1 containing less than 0.010 percent by weight sulfur.

4. The martensitic stainless steel of claim 1 containing about 0.23 percent carbon, 0.75 percent manganese,'0.35 percent silicon, 12 percent chromium, 0.80

percent nickel, 1.0 percent molybdenum, 1.0 percent tungsten, 0.25 percent vanadium, 0.29 percent niobium and the remainder substantially all iron. 

2. The martensitic stainless steel of claim 1 containing about 0.3 percent niobium.
 3. The martensitic stainless steel of claim 1 containing less than 0.010 percent by weight sulfur.
 4. The martensitic stainless steel of claim 1 containing about 0.23 percent carbon, 0.75 percent manganese, 0.35 percent silicon, 12 percent chromium, 0.80 percent nickel, 1.0 percent molybdenUm, 1.0 percent tungsten, 0.25 percent vanadium, 0.29 percent niobium and the remainder substantially all iron. 